Polyaniline (PAn) is stable in air and has a relatively low manufacturing cost. In addition, PAn has been found to be very useful in various applications, such as: material for making battery electrodes due to its oxidation-reduction properties, material for making pH sensor due to its proton exchange property, material for making a display element due to its electrochromic property, and electro-magnetic interference shielding and antistatic coating. Moreover, in 1990, Japan Bridgestone Co. of Japan [(U.S. Pat. No. 5,066,556 (1991); U.S. Pat. No. 4,957,833 (1990)] used polyaniline deposited on a current collector as working electrode in conjunction with a lithium electrode to produce a button battery with a discharge capacity of 80 Ahr/kg. Ricoh Co. of Japan [U.S. Pat. No. 4,999,263 (1991); U.S. Pat. No. 4,948,685 (1990)] used a polyaniline membrane of 0.05 mm thick deposited on a porous metallic membrane to produce a film-type battery with outside dimensions of 50 mm long, 50 mm wide and 0.9 mm thick. Sony Co. of Japan (Modern Plastics Int., Aug. (1991) 33) utilized polyaniline as an antistatic layer on a high density magnetic memory disc (4M).
Polyaniline (PAn) basically has a structure which can be represented by the following formula (Faraday Discuss Chem. Soc., 88 (1989) 317): ##STR1## wherein 0.ltoreq.y.ltoreq.1. It is termed leucoemeraldine, emeraldine and pernigraniline, when y is 0, 0.5 and 1, respectively.
A conjugated conducting polymer in general has a conductivity of about 10.sup.-12 .about.10.sup.-19 S/cm in the undoped state, which is increased to a value of about 10.sup.0 .about.10.sup.5 S/cm after the conjugated conducting polymer is oxidatively doped. Polyaniline (PAn) is different from other conjugated conducting polymers in that it can be doped to become a conducting form without requiring their .pi.-electrons to be changed through protonation by exposure to an appropriate protonic acid in aqueous solution [Chiang, J. C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193-205]. Yue and Epstein have synthesized a protonic acid self-doped PAn by reaction of emeraldine base with fuming sulfuric acid to give a sulfonic acid ring-substituted PAn having a conductivity of 0.1 S/cm; however, it is insoluble in the acid form (doped) and can be made soluble only when it is converted to a salt form (undoped) [Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800-2801. Yue, J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665-2671]. Genies et al. also have attempted to synthesize a self-doped PAn by reaction of emeraldine base directly with propane or butane sultone [Hany, P.; Genies, E. M.; Santier, C. Synth. Met. 1989, 31,369-378]. However, the reaction product has a very poor solubility and low conductivity (.about.10.sup.-9 S/cm). So far, all the self-acid-doped PAns reported in the prior art are insoluble, and thus their applications are limited. Dao and co-workers have synthesized a water soluble polyaniline, poly(aniline-propanesulfonic acid) (PAPSA), by reaction of leucoemeraldine base (a fully reduced PAn) with propanesultone [Bergeron, J. Y.; Chevalier, J. W.; Dao, L. H. J. Chem. Soc., Chem. Commun. 1990, 180-182]. However. the PAPSA is supposed to have no imine nitrogen (--N.dbd.) to allow self-acid-doping, although they have claimed the appearance of a strong absorption band at about 900 nm, which always appears for protonic acid-doped PAn. Recently, IBM reported that a water-soluble poly(aniline-co-N-(4-sulfophenyl)aniline) has been synthesized by copolymerization [Macromolecules 1994, 27, 3625-3631]. However, this product was found via elemental analysis that it is not self-acid-doped but is HCl-doped PAn. In addition, this PAn is only soluble in aqueous NH.sub.4 OH solution and is not soluble in water.